Conventional techniques for testing an electronic unit under test, or “UUT,” involve directing the UUT to generate an output signal, measuring the output signal, and determining whether the output signal is within acceptable limits. If the output signal falls within acceptable limits and no other failures occur, testing usually passes. Otherwise, testing may fail. UUTs may come in many varieties and are often provided in the form of semiconductor chips, devices, components, and assemblies, for example.
Some UUTs are designed to operate in the RF (Radio Frequency) and/or microware frequency ranges. Such UUTs may generate output signals at high frequencies, such as many megahertz or gigahertz. A conventional system for testing an RF or microware UUT includes a downconverter configured as a super-heterodyne receiver. For example, the downconverter includes a local oscillator that generates a high-frequency signal, generally referred to as a local oscillator signal, or simply “LO.” A mixer combines the LO with the test signal from the UUT to generate an output signal. The output signal from the mixer includes both an upper frequency band and a lower frequency band, which correspond to the sum and difference, respectively, of frequencies between the LO and the test signal. One or more filters may select one band (usually the lower band) and block the other band. An analog-to-digital converter (ADC) digitizes the filtered signal, e.g., by sampling the filtered signal at a sufficiently high rate to capture frequency content of interest. A processor obtains the samples from the ADC and performs a DFT (Discrete Fourier Transform). A test program analyzes the DFT to determine whether it meets acceptable limits, e.g., for spectral purity, signal-to-noise ratio, expected amplitudes at expected frequencies, and so forth. Given that systems often test UUTs for spectral purity and noise, a very pure and stable local oscillator is often used, such as a YIG (Yttrium-Iron-Garnet) oscillator.